This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Many biological functions are performed by protein complexes. With the development of modern proteomics technology, all the proteins in cells can be identified. The next step is to study the interactions of various proteins in terms of structure-function relationship. So far the popular approach is Q-TOF with the assistance of ion mobility technique to characterize those macromolecular assemblies in native form. Standard bottom-up proteomics is used to identify the constituent components after the whole assembly is denatured and enzymaticall digested. Various fragmentation methods, especially collisionally activated dissociation (CAD), employed in tandem mass spectrometry are used to break the whole assembly into smaller pieces to obtain insights into how the subunits interact with each other. We recently developed a top-down approach to obtain proteomics and structural biology information in one experiment based on the Bruker 12 tesla FTICR platform.